1. FIELD OF THE INVENTION
The present invention relates to an image forming apparatus such as a digital color copy machine and to a method therefor.
2. DESCRIPTION OF RELATED ART
Generally speaking, a known digital color copy machine is comprised of an image reader for reading a document by a color image sensor and converting color image signals read to gradation data and a printer for printing an image responsive to the gradation data by an electrophotographic method. In the printer, the gradation data are converted into binary print image signals by performing a binarizing processing to them and, thereafter, a laser diode is driven responsive to the print image signals to form an image by scanning the laser beam emitted from the laser diode on a photoconductive drum.
In the digital color copy machine of this type, dots, for example in a form of (N.times.N) matrix are made correspond to one picture element as one block in order to represent gradations of (N.sup.2 +1) and, in general, the dither method is widely used to reproduce half-tone images having much more gradations. However, this method has such a disadvantage in that the image resolution is lowered to increase the number of gradations due to an enlargement of the block assuming the dot size is constant and, to the contrast, the number of gradations has to be reduced in order to enhance the image resolution.
Also, pulse-width modulation method or intensity modulation method has been used as a method for representing gradations to one dot to be printed. In the pulse-width modulation method, so called triangular wave method has been utilized (see, for example, U.S. Pat. No. 4,763,199 and U.S. Pat. No. 4,782,398). In the method, the size of one dot is realized by varying the amount of exposure light of the laser beam to the photoconductive drum according to the print image signals.
In the digital color copy machine, an electrostatic latent image is formed on the photoconductive drum by driving the laser diode responsive to print image signals using the above intensity modulation method, the latent image is developed with toner to form a toner image and then the toner image formed is transferred on a copy paper. As well known to those skilled in the art, the relationship of the gradation degree (density) of the toner image to that of a document image becomes such a relationship as indicated by a reference numeral 91 in FIG. 4.
This characteristic is called .gamma.-characteristic wherein the gradient thereof becomes large in a range of the medium gradation and becomes small i.e. saturated in a range of the high gradation as indicated by a reference numeral 91a. Accordingly, thin characters of a document image are printed thinly and, upon copying a half-tone image such as a photograph having high gradations, the faithfulness of the printed image is lowered since the number of gradation is decreased substantially in the range of the high gradation as is apparent from the characteristic 91a.